The present invention relates generally to the field of digital imaging systems, such as x-rays systems employing digital detectors. More particularly, the invention relates to a technique for enhancing the dynamic range of a digital detector arrangement in an x-ray imaging system to provide uniform and useful image data of various tissues and patient morphologies.
Digital imaging systems have become increasingly important in a number of fields, particularly in the medial diagnostic field, and are often preferred over conventional techniques. For x-ray imaging, for example, conventional systems employ photographic film which is exposed during an x-ray examination, with contrast and details in the resulting image being provided by the various levels of absorption of the x-ray radiation passing through the patient. For example, bones and other similar tissues appear bright in the resulting photographic image, whereas soft tissues, such as the lungs appear darker. Challenges exist in balancing the parameters of exposure to provide the most useful resulting image, typically contrasting those features of particular interest. Similar challenges are presented in digital imaging systems.
In current digital x-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient""s tissues similar to those available through conventional photographic film techniques.
Digital x-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films were prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital x-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
Despite their utility in capturing, storing and transmitting image data, digital x-ray systems are still overcoming a number of challenges. For example, while film-based systems make use of light boxes or similar back lighting for viewing of the images, digital images are more often viewed on computer monitors or similar displays, which can suffer from a tendency to saturate regions of a reconstructed image, either by reproducing the regions as too bright or too dark. The underlying problem in such images may be rooted in the dynamic range of the image data, which may be mismatched to the dynamic range of an output device. Extended dynamic range techniques have been developed and implemented in an analog domain, such as in conventional image intensifiers and pickup tube systems. However, these same techniques are not easy to implement in the digital domain for use in displaying digital x-ray images.
The difficulty in systems employing digital imaging detectors stems from the digital dynamic range of the various components of the system. In particular, the dynamic range of the digital detector may be different from that of a display or output device. Thus, the dynamic range of the image data provided by the detector may need to be adapted to that of the output device. In one present system, for example, each pixel may be encoded in the data acquisition circuitry by 12 bits of data. A display device, however, may have a different dynamic range, typically reduced from the 12 bit range in the present example. In one presently available system, soft copy displays, such as computer monitors, are capable of providing a useful dynamic range of only 8 bits. Depending upon the type of image, the anatomy or tissues viewable in the image, and similar factors, improper adjustment of the dynamic range can result in loss of desired contrast, reducing the ability to distinguish features in certain areas (e.g., at a lower end of the dynamic range). Saturation of other regions of the image may also occur, with similar loss of detail in those regions (e.g., at an upper end of the dynamic range). Such problems have, been encountered particularly in chest x-rays, in which improper dynamic range adjustment can result in loss of contrast in regions of the heart, while producing saturation in lung regions of the reconstructed image.
There is a need therefore, for an improved technique for extending the useful dynamic range of a digital imaging system, particularly of an x-ray system employing a digital detector. Moreover, a present need exists for an improved system which can be implemented through programmable filtering to adapt the dynamic range applied to specific regions of an image to provide useful image data through contrast enhancement in the regions, while avoiding producing low contrast in darker regions or saturation in lighter regions. There is a particular need for a straightforward technique for adjusting the dynamic range of acquired image data so as to avoid loss of contrast and saturation of regions in a reconstructed image on an output device with a different dynamic range.
The invention provides a dynamic range extension technique adapted for use in digital imaging systems, and particularly in x-ray systems. The technique may be implemented via appropriate programming of signal processing or digital filtering components of the system, based upon input data collected from a digital detector. The technique may thus be implemented on existing systems, such as through software retrofits, as well as to new systems where the extended dynamic range is desirable. While the technique is particularly well suited to direct digital x-ray imaging systems, it may also find application in similar fields, such as in the display of images encoded from conventional supports, such as photographic film. Even more generally, the technique may find application in fields outside of x-ray imaging, and outside of the field of medical diagnostic imaging.
The technique employs a balance between pure gray level processing and pure frequency processing. Parameters employed for converting input data to output data thus balance a tendency to overlimit contrast through pure gray level processing, with a tendency to produce xe2x80x9cringingxe2x80x9d artifacts through pure frequency processing. The parameters may exhibit nonlinearities by which they vary over an input range as a function of the input signal value, and desired thresholds in the input signal values. By applying both the gray level processing control parameters and the frequency processing control parameters over the range of input values, the resulting dynamic range is extended to provide good contrast in darker regions of the resulting image, while avoid saturation of brighter regions.
In general, the technique permits processing to decrease individual pixel values or intensities in bright portions of a reconstructed image, while maintaining some level of detail in these regions. The original image data may thus be adjusted from a first dynamic range, such as a 12 bit dynamic range, to a desired range, such as an 8 bit range of a display. Contrast in darker regions of the image, such as in heart regions of a chest x-ray is therefore preserved, while saturation of the brighter portions of the image, such as in details of the lungs is avoided.